1. Field of the Invention
This invention relates to a fast, externally-controlled ferroelectric phase shift controller for coupling control of microwave cavities, including, but not limited to those used in linear colliders, superconducting linear and circular accelerators, energy recovery linear accelerators (ERLs) for free electron lasers and ion coolers, superconducting RF systems of circular accelerators and storage rings, and other particle accelerators, and the methods and systems required to carry out ferroelectric tuning and phase shift adjustment.
2. Background of the Technology
Currently, experiments involving sub-atomic particles generally take place using energetic beams generated in particle accelerators. Particle accelerators generally fall into one of two groups: linear particle accelerators and circular particle accelerators. In a linear particle accelerator, particles are accelerated in a straight line, with a target of interest at one end. In a circular particle accelerator, particles move in a circle until they reach sufficient energy. Circular particle accelerators have an advantage over linear accelerators in that the ring topology allows continuous acceleration without an end. Currently, the largest linear particle accelerator is the Stanford Linear Accelerator (SLAC), which is 3 kilometers long. The largest circular particle accelerator, by contrast, has a circumference of 26.6 kilometers.
A need exists in the art for fast ferroelectric components that control reactive power for fast tuning of cavities of superconductors utilized in particle accelerators, such as those to be used, for example, in the superconducting Energy Recovery Linac (ERL). This need will continue as the next generation of particle accelerators is constructed, for example, the International Linear Collider (ILC), which should fulfill the well recognized need in the art for a linear e+e− (electron-positron) collider with a center-of-mass energy Ecm between 0.5 and 1.0 TeV.
Further, fast electrically-controlled coupling is desirable for linear accelerators in order to match the cavity with the feeding transmission line as the beam load varies. Fast electrically-tuned amplitude and phase control with a feedback system is useful in order to be able to compensate for possible phase deviations of the input RF fields in each cavity. In a linear accelerator, RF fields in all cavities must have precisely-fixed phase differences with respect to one another, plus uniform amplitudes. As an example, this is especially critical for the proposed ILC design, which requires each klystron to drive 36 separate cavities.
The proposed ILC design specification is presented herein as an example of a superconducting linear accelerator which utilizes the ferroelectric phase shift controller of the present invention. This design is merely presented as one example of the type of particle accelerator that can be utilized in conjunction with an embodiment of the present invention. One skilled in the art will recognize that the present invention could be utilized in any number of particle accelerators, or in other applications which require fast phase shifting of RF power.
In 2004, the International Committee for Future Accelerators (ICFA) formed the International Technology Recommendation Panel (ITRP) to evaluate and recommend technology for the future ILC. In Sep. 2004, the ITRP selected the superconducting RF power technology as utilized in TESLA, which accelerates beams in 1.3 GHz (L-Band) superconducting cavities. In the selected concept, two main linear accelerators, each including approximate 10,000 one-meter long nine-cell superconducting cavities, will be used. Groups of 12 cavities will be installed in a common cryostat. The accelerating gradient is about 25 MeV/m and the center of mass energy is 500 GeV. The RF power is generated by about 300 klystrons per linear accelerator, each feeding 36 9-cell cavities. The required peak power per klystron is about 10 MW, including a 10% overhead for correcting phase errors during the beam pulse which arise from Lorentz force detuning and microphonics. The RF power pulse length is 1.37 ms, which includes a beam pulse length of 950 μs, and a cavity fill time of 420 μs. The repetition rate is 5 Hz. The average mains power consumed by the system at 500 GeV center-of-mass energy is thus about 70 MW, assuming an RF power source efficiency of approximately 65%, and a modulator efficiency of about 85%. Refrigerators used to cool the structure will require an additional 8.5 MW, to dissipate heat from RF power losses in the structures.
In order to successfully power the design, there is a need in the art for an external fast phase shift controller which will allow quick extraction of RF power from the superconducting sections after the RF power pulse ends, thereby decreasing the cavity heating and the refrigerator power consumption.
Ferrite tuners were originally suggested for this application, such as those being developed at CERN for the Superconducting Proton Liner Accelerator. These tuners are designed to provide fast phase and amplitude modulation of the drive signal for individual superconducting cavities. The tuner is based on two fast and compact high-power ferrite phase shifters magnetically biased by external coils. However, the tuning frequency for this device has an upper cut-off at 2 kHz that comes mainly from the remaining eddy currents inside the RF power structure. Thus, its shortest switching time is about 1 millisecond. For applications such as those discussed above, switching times must not exceed 50-100 microseconds. Accordingly, there is a need in the art for faster ferroelectric phase shift controller.
There is a further need in the art for an external fast phase shift controller which will stabilize the necessary precise phase differences between cavities in near-real-time. This compensates for fluctuations in the phase difference in each cavity due to microphonics and Lorentz-force cavity distortions.
Recently, ferroelectric devices for fast switching applications have received close attention, and are already used in low- to moderate-power military and communications systems as fast tunable components, because they have the ability to operate up to frequencies above 30 GHz with reasonably low loss, and have high intrinsic tuning rates. Ferroelectrics have an E-field-dependent dielectric permittivity ∈ (E) that can be very rapidly altered by application of a bias voltage pulse. The switching time in most instances would be limited by the response time of the external electronic circuit that generates and transmits the high-voltage pulse. The minimal switching time achieved in operating devices is less than one nanosecond. There is accordingly a need in the art for a ferroelectric material with good working properties for use in high-power RF switches for linear collider applications.